Breast cancer is the most common cancer among U.S. females and is the second leading cause of death among women. In the last decade, the incidence of breast cancer has increased at a rate of approximately 2% per year. There is speculation that environmental chemicals, especially ecoestrogens, may be responsible for some of these cancers. We have hypothesized that estrogenically-active chemicals can exert predisposition for or against breast cancer, depending on the xenobiotic and timing of exposure. We have recently demonstrated that diethylstilbestrol and genistein given neonatally alter cell differentiation and cell proliferation in mammaries of 50 day old female rats and that these two estrogenically-active xenobiotics increased the Latency and decreased the incidences and multiplicities of dimethylbenz(a)anthracene (DMBA)-induced mammary tumors (22,23). This is to be contrasted to prenatal diethylstilbestrol exposure increasing tumor multiplicity in the mammaries of rats exposed to DMBA (29). These data support the theory of developmental windows and estrogenically-active xenobiotics playing a role in predisposition for mammary cancer. It is our objective to investigate the potential of six environmental chemicals during three critical periods of development for altering susceptibility for breast cancer and to investigate two mechanisms that could be predictive indicators of this disease. Sprague Dawley CD rats will be exposed during the prenatal, neonatal and pubertal periods of development to diethylstilbestrol, genistein, l-(o-chlorophenyl)-l-l)p- chlorophenyl)-2,2,2-trichloroethane (o,p'-DDT), Aroclor 1221, Aroclor 1254 and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We will subsequently study cell differentiation in mammary whole mounts and measure cell proliferation using proliferating cell nuclear antigen as a marker of mitotic activity. The maturation of undifferentiated terminal end buds to more differentiated lobules have been shown to yield structures that are less susceptible to DNA damage and replication (2). Likewise, cells that have a higher proliferation index are more susceptible to carcinogens. We will also investigate the effects of these environmental chemicals on pituitary, ovarian and uterine weights as markers of estrogen action and on circulating levels of estrogen, progesterone, prolactin and growth hormone. Finally, we will investigate the potential of selected xenobiotics and developmental windows to alter latency, incidence and multiplicity of DMBA induced adenocarcinomas.